Recently, a power generation device, such as a solar power generation device, a fuel cell or a wind power generation device, increasingly has been installed at electric power consumer sides, such as factories or houses. Further, a grid connected system also has been put to practical use. The grid connected system makes this kind of power generation device perform the grid connected operation with an electric power system of a commercial power source, thereby using the power generation device as a dispersed power source. In the grid connected system, surplus electricity, which is obtained by subtracting the amount consumed in a consumer side from the amount of electricity generated in the dispersed power source, is allowed to flow into the electric power system of the commercial power source by reverse power flow. Then, when the surplus electricity is allowed to flow from the consumer side to the electric power system by the reverse power flow, there is also the case where the consumer gets paid for the amount of the surplus electricity allowed by the reverse power flow from an electric power provider that provides electricity as the commercial power.
When the number of consumer sides installing the dispersed power sources is increased and a plurality of dispersed power sources allow electricity to flow into the electric power system by the reverse power flow, there is a possibility that a voltage of the electric power system is increased, a frequency in the electric power system is changed due to load reduction or phase-lead of the system voltage is generated. Especially, when the number of dispersed power sources connected to the electric power system increases, such a phenomenon is easily generated. Furthermore, the longer a distance of a pole transformer to the dispersed power source is, the more the voltage of the electric power system is easily increased due to the reverse power flow performed by other dispersed power source located closer to a pole transformer.
In order to prevent the above-mentioned problems generated in the case where the grid connected system makes the dispersed power source perform the grid connected operation with the electric power system, the dispersed power source has an output suppression function that suppresses the electricity flowing into the electric power system when the voltage of the electric power system increases upon the reverse power flow. Also, as described above, the longer the distance of the pole transformer to the dispersed power source is, the more the voltage of the electric power system is easily increased, and the more the output suppression function also get executed easily. Thus, there has been a difference between frequencies of the output suppression controls performed at consumer sides installing the dispersed power sources, or a difference between the amounts of the electricity allowed to flow by the reverse power flow at consumer sides. In order to correct such an inequality generated among the consumer sides, there has been proposed a technique, in which the longer a distance of a pole transformer to each dispersed power source is, the higher the system voltage at which the output suppression control starts is set, thereby reducing the difference between frequencies of the output suppression controls generated (see Japanese Patent Publication No. 4266003).
As described above, the phenomena, such as increasing of the system voltage, change of the frequency and the phase-lead, are monitored in each dispersed power source. Then, when the phenomena are generated beyond prescribed volumes, the reverse power flow into the electric power system is suppressed, thereby reducing the effect on the electric power system. Also, the technique described in the above-mentioned document sets the voltage at which the output suppression control starts, based on the distance of the pole transformer to the dispersed power source, thereby equally allocating the opportunity of the reverse power flow to each dispersed power source. In other words, the technique can equally allocate the potential of selling the surplus electricity regardless of the distance of the pole transformer to the dispersed power source.
Then, according to the technique described in the above-mentioned document, each dispersed power source sets a threshold value for the system voltage at which the output suppression control starts, based on impedance between the pole transformer and the dispersed power source (that is, based on the distance), thereby correcting the inequality with respect to the opportunity of the reverse power flow into the electric power system. Therefore, as the number of the dispersed power sources connected to the secondary side of the pole transformer is increased in neighborhood with houses lined so close together, a difference between threshold values set for adjacent dispersed power sources is reduced. Thus, in regard to setting of the threshold value, high accuracy is required.
When setting the above-mentioned threshold value, each dispersed power source also needs to calculate the impedance between the pole transformer and the dispersed power source connected to the secondary side of the pole transformer. Therefore, when the dispersed power source is installed, acquiring of information regarding a distribution network topology, a wiring distance and a line type is needed, and further, information regarding spec on the dispersed power source is also needed. That is, the management of a lot of information is needed, and therefore, there has been a problem that the cost of data management is increased.
As explained above, the technique described in the above-mentioned document has a problem that it is difficult to be implemented under conditions that there is a large number of the dispersed power sources connected to the secondary side of the pole transformer and there is a small difference between distances of cable runs in the electric power system among the adjacent dispersed power sources.
Then, as explained above, when the reverse power flow is suppressed for the convenience of the electric power provider side, the surplus electricity is not allowed to flow into the electric power system by the reverse power flow even if the surplus electricity is generated at the consumer side. That is, because the consumer can not get paid for the amount of the surplus electricity from the electric power provider by performing the reverse power flow of the surplus electricity, such suppression causes economic damage to the consumer. Therefore, the consumer needs an extended period to recoup the cost invested in the installation of the dispersed power source, and the cost-effectiveness is reduced. This contributes to preventing of the diffusion of the dispersed power source.
Further, when the output suppression control is performed at the consumer side, the amount of electricity generated in the dispersed power source decreases due to the output suppression control despite essentially the state where the dispersed power source can generate power. As a result, there is a problem that the operational efficiency of the dispersed power source is reduced. Especially, the solar power generation device converts solar energy into electric energy, and fuel expenses are not needed. Thus, it is not economical to suppress the power generation despite the state where the dispersed power source can generate power. Accordingly, there has been also a possibility to reduce the effect of a subsidy system directed at the diffusion and promotion of the solar power generation device.
So, there has been also considered a large energy storage system installed in the electricity network by the electric power provider. In this case, the large energy storage system stores the electricity flowing from the dispersed power source by the reverse power flow, thereby preventing the instability of the system voltage and reducing the frequency of the output suppression control performed at the dispersed power source side. However, it is expected that the energy storage system is very expensive to install, and the electric power provider does not benefit much economically. Also, it is difficult to secure a place for the installation of the energy storage system in a residential area. Therefore, there is a high possibility that such a large energy storage system does not enable the diffusion of the dispersed power source.